The invention relates to a method for producing an adhesion-mediating coating on the surface of workpieces, preferably those of headlight reflectors made of plastic, having a vacuum tank in which the workpieces are exposed to a plasma coating process, with inlet and outlet openings in the tank, by which a process atmosphere determining the plasma coating process can be produced, during a controlled inlet and outlet of substances.
From DE-A 40 10 663, an apparatus and method are known for the production of coatings on the surface of workpieces, preferably headlight reflectors made of plastic. It involves especially a special surface treatment of three-dimensional substrate surfaces, the object being to achieve a corrosion-resistant and nonsmearing mirror surface.
What is involved is a PCVD coating process with a microwave-ECR plasma coating source and a so-called rotary cage situated in the vacuum chamber and carrying the substrates past the coating source in a planetary motion controlled by frequency and phasing.
The coating process provides a plurality of process steps which first modify the substrate on its surface within the scope of a noncoating plasma pretreatment so that functional groups will form on it, such as hydroxyl-carbonyl or amino groups. After that, for improved adhesion mediation, a coating formed from a SiC or SiCO gas atmosphere is deposited onto the substrate surface. This is followed by the application of an aluminum coating to the substrate. In a final step, a protective coating of the same coating composition as the above-described adhesion mediating layer is applied to the metallized substrate to increase its resistance to corrosion and smearing.
Further testing has shown, however, that many substrate materials, such as plastics and varnishes, undergo irreversible damage to the substrate surface under the process conditions described in DE-A 40 10 663 This is especially true of the first monolayers of plastic substrates, where damage is caused by ultraviolet rays occurring in the plasma and by collision of high-energy plasma particles with the substrate surface. Such disadvantageous interactions with the substrate surface depend on the types of particles included, and on the distance between substrate and plasma, as well as on the prevailing particle partial pressures.